The present invention relates to power units comprising an internal combustion engine supercharged by a turbine-compressor unit which receives driving energy from the exhaust gases of the engine.
The name "internal combustion engine" will be understood as designating any such engine having variable volume chambers, such as a spark ignition or compression ignition engine, having at least one reciprocating or rotary piston.
The invention more particularly relates to those engines whose ignition does not require that the air admitted into the chambers be pressurized, except under exceptional conditions of use (starting during very cold weather for instance) and which consequently can operate with "natural" induction.
Most power units of that type in which the energy required for the turbine-compressor unit is supplied exclusively by the exhaust gases of the engine (i.e. those units which have no auxiliary combustion chamber) have shortcomings due to the fact that the energy supplied by the exhaust gases decreases as the engine becomes unloaded. For a large range of operating speeds, this energy is insufficient, so that the power and the torque of the unit are substantially the same as those of a non-supercharged engine. Acceleration from a speed within that range is very slow because of the considerable time for the turbine-compressor unit speed to increase to a value at which it is effective.
Different solutions have been proposed to overcome the difficulty. It has been suggested to use a turbo-compressor unit which supplies an appreciable supercharging pressure for all operating speeds from a speed which is about half the rated speed and to provide a waste valve in the supercharging air circuit. The waste valve opens when the supercharging pressure exceeds a pre-determined value, selected for example with respect to the maximum combustion pressure which the engine may withstand. The simple fact that the range of speeds at which the installation operates practically without supercharging extends practically to half the rated speed shows that this solution is not satisfactory.
A power unit has also been proposed comprising an internal combustion engine capable of operating with natural induction, supercharged by a turbine-compressor unit receiving its energy solely from the exhaust gases of the engine, the compressor supplying the engine with air through an inlet pipe fitted with an inlet for atmospheric air, closed off in on-load operation, a member for adjusting the power of the engine movable between a maximum power position and an idling position, and a valve placed in the air inlet of the compressor and movable between an open position and a closed position, biased towards closing by the power adjustment member when this latter is brought to its idling position, at least beyond a pre-determined position. In that prior art unit, the power controlling member is a throttle member placed in the air intake to the engine and the air inlet valve is controlled by the position of the throttle member. That solution is not completely satisfactory since the compressor is partially or totally out of action during operating conditions in which the engine speed remains sufficient to ensure normal operation of the compressor. For example, such conditions occur during deceleration at high speed, when the engine operates as a retarder. Moreover, the arrangement cannot be used with compression ignition engines in which the amount of fuel admitted into the combustion chambers of the engine is directly metered the combustion always taking place with a considerable excess of air.
It is an object of the present invention to provide a power unit of the kind defined above, but in which the normal conditions of air intake to the turbo-compressor are maintained as long as is necessary to avoid its slowing down.
According to an aspect of the invention, there is provided a power unit comprising an internal combustion engine capable of operating without supercharging and associated with a turbine-compressor unit whose turbine is connected to receive the exhaust gases of the engine and whose compressor is connected to supply compressed air to an intake pipe of the engine which is provided with a non-return check valve adapted to admit atmospheric air into said intake pipe in response to occurence of a subatmospheric pressure in said intake pipe, a compressor inlet valve located in the air inlet of the compressor and movable between an open position and a closed position, operatively connected to a power control member movable between a maximum power position and an idling position, for controlling the power delivered by the engine, said control member biasing the inlet valve toward closure when it is moved towards its idling position, at least beyond a pre-determined position, and toward full opening when it is moved towards its maximum power position, locking means arranged to prevent closure of said compressor inlet valve, and control means subjected to the air pressure delivered by the compressor and operatively associated with said locking means for rendering said locking means effective in response to the outlet pressure of the compressor being greater than a predetermined threshold.
According to another aspect of the invention, there is provided a power unit comprising an internal combustion engine capable of operating without supercharging and associated with a supercharging turbine-compressor unit whose turbine is connected to receive the exhaust gases of the engine and whose compressor is connected to supply compressed air to an intake pipe of the engine which is provided with a non-return check valve adapted to admit atmospheric air into said intake pipe in response to occurence of a subatmospheric pressure in said intake pipe, a compressor inlet valve located in the air inlet of the compressor and movable between an open position and a closed position, operatively connected to a power control member movable between a maximum power position and an idling position for controlling the power delivered by the engine, a power control member biasing the inlet valve towards closure when it is moved towards the idling position, at least beyond a pre-determined position, and positively opening the inlet valve when it is moved towards its maximum power position, and a by-pass provided with a second non-return valve connecting the outlet of the compressor to the inlet of the turbine, in parallel flow relationship to the engine, having a sufficiently large cross-sectional area for the air supplied by the compressor which is not absorbed by the engine to flow through the by-pass pipe with a pressure drop sufficiently low for avoiding surge of the compressor.
The installation may further comprise a waste gate, typically placed in the exhaust pipe of the engine, upstream of the inlet of the turbine. This valve is adjusted to open and discharge exhaust gases directly to the atmosphere when the supercharging pressure exceeds a predetermined maximum value chosen for example with respect to the maximum combustion pressure which the engine can withstand.
The power unit thus operates with supercharging in all conditions in which the exhaust gases have sufficient energy to cause the turbine-compressor unit to operate normally, whereby a disadvantage of prior systems is overcome. In the latter, the closing of the inlet to the compressor takes place in response to a single parameter, the position of the throttle member and for a predetermined position of that member. The choice of this position is a compromise which must take into account numerous parameters, particularly the temperature and the pressure of the atmospheric air, the temperature of the engine, the temperature of the exhaust gases . . . So, unless complex correction mechanisms (not mentioned in the above-mentioned prior art documents) are provided to regulate at all times the definite position of the throttle member with respect to all these parameters, the closing of the air inlet of the compressor takes place frequently whereas the energy contained in the exhaust gases would still be sufficient for the turbine-compressor unit to ensure the supercharging of the engine.
In an advantageous embodiment of the invention, a by-pass pipe of large cross section communicates the output of the compressor with the inlet of the turbine so as to avoid compressor surge during sudden slowing down of the latter and jerks in operation.
The invention will be better understood from the following description of particular embodiments thereof, given by way of examples. The description refers to the accompanying drawings.